Titanium oxide (TiO2) is a semiconductor substance with a wide array of industrial applications. One particularly useful property of titanium oxide is its ability to be a photocatalyst when present in the anatase phase, promoting oxidation-reduction (redox) reactions when irradiated with ultraviolet light. Titanium oxide has been investigated for applications including commercial water purification, air purification, hydrogen production by photochemical splitting of water, nitrogen fixation, odor control, antimicrobial surface coatings, self-cleaning surface coatings, and dye-sensitized solar cells.
When semiconductors such as titanium oxide are in the form of nanoparticles, their physical and chemical properties may be strongly dependent on the size, shape and crystallinity of the nanoparticles. For example, nanofibers and nanorods of titanium oxide tend to have increased photocatalytic activity relative to nanospheres. One possible explanation for this is an increased delocalization of photo-induced electrons (e−) and holes (h+) along the length of the nanofiber or nanorod surface, resulting in a decrease in the probability of e−/h+ recombination at surface defects relative to that of nanospheres. In addition, although titanium oxide can exist in a number of structure types, the anatase structure type (anatase phase) is by far the most photocatalytically active phase.
Conventional nanoparticles containing titanium oxide have a number of disadvantages. Non-spherical titanium oxide nanoparticles have been expensive to produce and difficult to obtain in large enough quantities for use in industrial or consumer applications. In addition, conventional non-spherical titanium oxide nanoparticles are in the rutile crystal phase, which is much less active photocatalytically than the anatase phase. Nanoparticles prepared by growth of titanium oxide in a physical template tend to have relatively large dimensions, which can limit their photocatalytic activity. Direct chemical synthesis of titanium oxide nanoparticles tends to produce complex titanates containing residual organic groups or other dopants that interfere with the desirable chemical and physical properties of the titanium oxide.
Non-spherical nanoparticles of titanium oxide in the anatase phase having controllable dimensions would be beneficial. Ideally, such nanoparticles would have dimensions small enough to provide useful levels of photocatalytic activity. It would be desirable to form such nanoparticles by a method that is relatively inexpensive and that can be readily scaled up to produce large quantities.